EXPERIMENTAL TRACKWAYS OF ISOPODS AND MILLIPEDES UNDER SUBAERIAL AND SUBAQUEOUS CONDITIONS: PALEOENVIRONMENTAL IMPLICATIONS FOR ICHNITES IN FINE-GRAINED SANDSTONE

Fossilized invertebrate traces (ichnites) are found in many sandstones on the Colorado Plateau (USA). Invertebrate ichnites from the fine-grained Coconino Sandstone (Permian) were interpreted by early workers as terrestrial arthropod traces. That conclusion was based on limited experiments conducted during the 1930’s and 1940’s with millipedes and isopods solely under subaerial conditions, which showed a loose resemblance between extant and fossil traces on dry sand. Here we present the first part of a new qualitative and quantitative analysis, which compares experimental invertebrate traces with simple fossil trails and trackways from the Coconino Sandstone and other similar sandstones. Neoichnological trackway experiments (178 total runs) were conducted using three types of invertebrates: common woodlice (Oniscus asellus), Florida ivory millipede (Chicobolus spinigerus), and the colored train millipede (Euryurus leachii). Traces were made on fine-grained sand under ≤12 experimental combinations of moisture level (dry, damp, wet, and subaqueous) and slope (flat, shallow, and steep). Wet sand transitioned from soupy to firm as surficial water content decreased upslope. Invertebrates crawling on soft to firm sand produced simple to compound, biserial trackways. However, trails were produced only in soupy-wet sand. If the common fossil trail Diplopodichnus biformis Brady, 1947 was indeed made by terrestrial homologous arthropods, then at least the trail-bearing surfaces in the Coconino Sandstone and other formations were probably soupy-wet at the time of trail formation. As for trackways with distinct imprints (e.g. Oniscoidichnus filiciformis Brady, 1949), comparative statistical analyses (e.g. Gower similarity index and logistic regression) carried out using binary characteristics and ratios of measurements may suggest that sandstones once interpreted as continental arid deserts likely had more complex paleoenvironments in which some facies were deposited by subaqueous processes.